Ultrasonic motor

ABSTRACT

An ultrasonic motor is provided in which a detector will detect the amplitude of a traveling wave generated on the stator. An oscillator approximates frequency of the travelling wave with respect to the resonance frequency of the stator while the amplitude of the traveling wave is decreased. If the oscillator approaches the frequency of the traveling wave to the resonance frequency of the stator, the amplitude of the traveling wave can be maintained against increasing pressure of the increased torque load of the ultrasonic motor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an ultrasonic motor which rotates a rotor bydeveloping a traveling wave on a stator.

2. Description of the Related Art

Conventionally, an ultrasonic motor which utilizes traveling waves iswell known in the art. The conventional motor has a piezoelectricvibrator which is adhered to a stator. The traveling wave is generatedon the stator while the piezoelectric vibrator is oscillating. The rotorrotates in response to the traveling wave. For example, the conventionalultrasonic motor is disclosed in Japanese Laid Open Patent PublicationNo. 58-148682 published on Sept. 3, 1983.

Referring now to FIGS. 7 and 8, a conventional ultrasonic motor isdisclosed. A rotor 72 and a stator 73 are contained in the casing orhousing 71.

The rotor 72 is connected to the spindle 74 which is rotatably supportedin the casing 71. The rotor 72 integrally rotates with spindle 74. Apair of piezoelectric vibrators 75a, 75b are adhered to the stator 73.The piezoelectric vibrators 75a, 75b generate a traveling wave on thestator 73.

A pressure applying mechanism 76 is provided between the casing 71 andthe rotor 72. The pressure applying mechanism 76 includes a rotor cam76a and a spindle cam 76b. The rotor cam 76a is fixed to the rotor 72.The spindle cam 76b is fixed to the spindle 74. Each cam 76a, 76b has aV-shaped bottom. A plurality of steel balls 76c is pinched or securedbetween the V-shaped bottoms.

The steel balls 76c are positioned at the bottom of the cams 76a, 76b.However, the steel balls 76c move from the V-shaped bottom in responseto an increase in load and generate an axial pressure with respect tothe spindle 74. Thus, pressure between the rotor 72 and the stator 73 isapplied due to the increase of load. The torque which is generated bythe rotor is transmitted to the spindle 74 due to the applied pressure.

However, in the conventional ultrasonic motor, the vibration from thepiezoelectric elements is confined due to the applied pressure since thepressure applying mechanism 76 generates a large pressure with respectto the increase of the load. Therefore, the conventional ultrasonicmotor rapidly reduces rotational speed in response to the increase ofthe load.

SUMMARY OF THE INVENTION

Accordingly, one of the objects of the present invention is to obviatethe above conventional drawbacks.

Another object of the present invention is to maintain the rotationalspeed despite an increase of the load.

To achieve the above objects and in accordance with the principles ofthe invention as embodied and broadly described herein, the ultrasonicmotor has a rotor and a stator which includes a spring arrangement forpressing the rotor to the stator with a predetermined pressure, amodulator for increasing the pressure in response to an increase ofload, and a detector for detecting intensity of a traveling wavegenerated on the stator. An oscillator approximates the frequency of thetraveling wave to the resonance frequency of the stator while theintensity of the traveling wave is decreased.

The intensity of the traveling wave increases when the frequency of thetraveling wave approaches the frequency of the stator. Therefore, if theoscillator matches the frequency of the traveling wave to the resonancefrequency of the stator, the intensity of the traveling wave can bemaintained against increasing pressure. Thus, the rotational speed ofthe rotor can be maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention andserve to explain the principles of the invention. Of the drawings:

FIG. 1 is a cross-sectional view of an ultrasonic motor showing a firstembodiment of the present invention;

FIG. 2 is a perspective view of a pressure modulating mechanism of thefirst embodiment of the present invention;

FIG. 3 is a block diagram of an electronic circuit of the firstembodiment of the present invention;

FIG. 4 is a flow chart of a program for executing the electronic circuitshowing in FIG. 3;

FIG. 5 is a graph showing the relationship between rotational speed andload of the first embodiment of the present invention;

FIG. 6 is a cross-sectional view of an ultrasonic motor showing a secondembodiment of the present invention;

FIG. 7 is a cross-sectional view of a conventional ultrasonic motor; and

FIG. 8 is a cross-sectional view of a conventional pressure applyingmechanism showing in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the present preferred embodimentof the invention, an example of which is illustrated in accompanyingdrawings.

Referring now to FIG. 1, the elements of an ultrasonic motor will beexplained. A housing 15 is screwed into or otherwise connected to abase 1. A pair of bearings 16, 17 are fixed to the housing 15 and thebase 1, respectively. A spindle 13 is rotatably supported by thebearings 16, 17.

An inner portion 2a of a ring shaped stator 2 is engaged with thebase 1. The stator 2 is fixed rigidly to the base 1 by screws 3 so as toprevent any vibration from being generated on the inner portion 2a. Aring shaped piezoelectric vibrator 4 is adhered or otherwise secured toone side of the stator 2. The piezoelectric vibrator 4 includes a pairof vibrator elements for generating the traveling wave on the stator 2and a detecting element for detecting amplitude of the traveling wave.Three conducting wires 5a, 5b, 5c are soldered to each of the elementsof the piezoelectric vibrator 4. The wires 5a, 5b and stator 2 are usedfor supplying the electric power to the vibrator elements of thepiezoelectric vibrator 4. The wire 5c (not shown) is used fortransmitting the voltage V to the control circuit 30. The voltage Vcorresponds to the amplitude of the traveling wave.

The stator 2 has an inner portion 2a, a vibrating portion 2c and aconnecting portion 2b integral with each other. The vibrating portion 2chas a plurality of equally pitched or spaced projections 2d along thecircumference of the stator 2. The stator 2 is made from aphosphor-bronze material so as to connect the stator 2 to earth orground. The stator 2 is electrically connected to the base 1.

The piezoelectric vibrator 4 is a well-known element which generates thetraveling wave on the stator 2. The piezoelectric vibrator 4 generatesthe traveling wave on the stator 2 when current having a 90° phasedifference is applied to the piezoelectric vibrator 4 through theconductive wires 5a, 5b.

The rotor 6 is pressed against the stator 2 at all times by the pressureof a disc spring 7. A stabilizer 8 is interposed between the rotor 6 andthe disc spring 7 so as to apply the pressure of the spring uniformlyaround the circumference of the rotor 6. A rubber sheet element 9 ispinched between the disc spring 7 and the stabilizer 8. A further rubbersheet 10 is held between the stabilizer 8 and the rotor 6. The rubbersheets 9, 10 prevent noise generation between the disc spring 7, thestabilizer 8 and the rotor 6.

The rotor 6 has an outer portion 6a, an inner portion 6c and aconnecting portion 6b integrally connected with each other. A frictionalfilm 11 is adhered or otherwise secured to one side of the outer portion6a of the rotor 6. The frictional film 11 is retained between the outerportion 6a of the rotor 6 and the vibrating portion 2c of the stator 2.

According to the present embodiment, the stabilizer 8 is made of steelso as to have a greater mass than the aluminum rotor 6. The steelstabilizer prevents vibrations on the rotor 6 from being transmitted tothe disc spring 7. Thus, the pressure of the disc spring 7 isstabilized. Further, the stabilizer 8 is preferably a rigid body so asto reduce the absorption of the vibrations on the rotor 6.

The stabilizer 8 has a ring shaped projection 8a. The projection 8a islocated radially inwardly from the contacting portion where the rotor 6is in contact with the stator 2. The rotor 6 is pressed against thestator by the projection 8a. The projection 8a distributes the pressureuniformly along the circumference thereof since the projection 8a is ofconstant height.

The disc spring 7 is supported by a guard portion 12a of a holder 12.The spindle 13 is rotatably positioned in the holder 12. The holder 12is rotatable around the spindle 13.

FIG. 2 is an exploded perspective view showing a pressure modulatingmechanism. A cam surface 12b is formed on the holder 12. The cam surface12b has four V-shaped lobes along bottom portion thereof.

A guard portion 13a is formed integrally with the spindle 13. The guardportion 13a has a cam surface 13b corresponding to the cam surface 12b.Four steel balls 14a, 14b, 14c, 14d corresponding in number to thenumber of lobes are pinched or held between the cam surfaces 12b, 13b.Torque is transmitted from the rotor 6 to the holder 12 through thestabilizer 8 and the disc spring 7. Further, the torque is transmittedfrom the holder 12 to the spindle 13 through the steel balls 14a, 14b,14c, 14d which are held between the cam surfaces 12b, 13b.

The pressure modulating mechanism comprises the cam surfaces 12b, 13band the steel balls 14a, 14b, 14c, 14d and converts the torque load intoan axial displacement of the holder 12.

When the torque load is small, the steel balls 14a, 14b, 14c, 14d arelocated on the V-shaped bottom portions of the cam surfaces 12b, 13bsince angular deviation between the holder 12 and the spindle 13 iscorrespondingly small. At this time, the holder 12 is displaced awayfrom the stator 2. Accordingly, the rotor 6 is pressed against thestator 2 under low pressure.

When the torque load is high, the steel balls 14a, 14b, 14c, 14d climbor otherwise move toward the top of the cam surfaces 12b, 13b, due tothe large angular deviation between the holder 12 and the spindle 13. Atthis time, the holder 12 is displaced in the direction of the stator 2.Accordingly, the rotor 6 is pressed against the stator 2 with increasedor higher pressure.

When the rotor 6 is pressed against the stator under the increasedpressure, the friction force between the friction film 11 andprojections 2d is increased. Thus, sliding or slippage between the rotor6 and the stator 2 can be reduced so as to increase output torque.

When the rotor 6 is pressed to the stator 2 under increased pressure,oscillation of the stator 2 is confined by the rotor 6. As a result ofthis, amplitude, (i.e., energy) of the traveling wave which is generatedon the stator 2 is decreased and the output torque is not satisfactorilyincreased. In the present invention, a control circuit 30 is availableto maintain the amplitude of the traveling wave.

FIG. 3 illustrates the control circuit 30. The control circuit 30comprises a power supply 31, an input interface 32, a microprocessor 33,a D/A converter 34, a voltage controlled oscillator 35, a phase shifter36, driving circuits 37, 38, boosting transformers 39, 40 and asmoothing circuit 41.

The power supply 31 is connected to a battery BTT. The power supply 31provides regulated electric power to the control circuit 30.

The input circuit 32 is interconnected between the micro-processor 33and a switch SW. The micro-processor 33, through the input circuit 32,is informed whether the switch SW is on or off.

The piezoelectric vibrator 4 includes a pair of electrodes 4a, 4b forgenerating the traveling wave on the stator 2. A pair of electricalinputs provide a current with a 90° phase difference to the electrodes4a, 4b. The amplitude of the traveling wave on the stator 2 can becontrolled by the voltage controlled oscillator 35. In more detail, theamplitude of the traveling wave can be increased by the frequency of thevoltage controlled oscillator 35 approaching the resonance frequency ofthe stator 2. Conversely, the amplitude of the traveling wave can bedecreased as the frequency of the voltage controlled oscillator 35 ismoved further away from the resonance frequency of the stator 2. Theresonance frequency of the stator will vary dependent on the pressureapplied thereto by the rotor 6 due to the pressure of spring 7. However,if the frequency of the oscillator 35 becomes equal to the resonancefrequency of the stator, the stator 2 and vibrator 4 can be damaged dueto the large stresses set up by the equal frequencies. Accordingly, itis desirable for the frequency of the oscillator to approach as closelyas possible to the resonance frequency of the stator but not so close asto equal or exceed the resonance frequency of the stator.

The voltage V is generated on the detecting electrode 4c by thetraveling wave. The level of the voltage V is in proportion to theamplitude of the traveling wave. The voltage V generated on thedetecting electrode 4c is regulated by the smoothing circuit 41. Thesmoothing circuit 41 receives an AC signed from the electrode 4c. Thecircuit 41 includes a rectifier circuit which converts the AC signal toa DC signal and a low pass filter. The micro-processor 33 reads theamplitude of the traveling wave through the smoothing circuit 41.

The micro-processor 33 controls the D/A converter 34 in response to thesignal which supplied from the smoothing circuit 41. Further, themicroprocessor 33 adjusts the frequency of the voltage controlledoscillator 35 so as to maintain a desired amplitude of the travelingwave.

FIG. 4 illustrates a program to be executed in the micro-processor 33.The micro-processor 33 starts the program when the power supply 31 isconnected to the battery BTT.

In step S1, initialization is executed so as to properly process theprograms.

In step S2, the micro-processor determines whether the switch SW is onor off. The step S2 is executed repeatedly until the switch SW is turnedon.

In step S3, a control flag f is defined by a code corresponding to theproper driving frequency f_(o).

In step S4, the micro-processor 33 supplies the code which is stored inthe control flag f to the D/A converter 34. As soon as the control codeis received by the D/A converter 34, the voltage controlled oscillator35 oscillates at the proper driving frequency f_(o). At this time, therotor 6 starts rotating.

In step S5, the micro-processor 33 determines the voltage V which isgenerated on the detecting electrode 4c.

In step S6, the voltage V is compared with reference voltage V_(o). Ifthe voltage V is equal to the reference voltage V_(o), themicro-processor 33 executes the step S8. If the voltage V is not equalto the reference voltage V_(o) the micro-processor 33 executes step S7.As discussed, if the frequency of the voltage controlled oscillatorapproaches too close to the resonance frequency of the stator 2, a largeamplitude may damage the vibrator 4 and stator 2. Accordingly, theoutput voltage from the sensor 4C is determined to be a referencevoltage V_(o) so as to be a feedback control for the vibration of thestator 2. Conventional ultrasonic motors have a constant output voltagesignal from sensors of the type identified at 4C even if the torque loadof the motor varies. That is, the frequency of the voltage controlledoscillator is not responsive to a change of the torque load in aconventional motor.

In step S7, the micro-processor 33 increases or decreases the code ofthe control flag f so as to match the voltage V with the referencevoltage V_(o).

In step S8, the micro-processor determines whether the switch SW isturned on or off. If the switch SW is on, the micro-processor 33executes step S4 again. If the switch SW is off, step S9 is executed.

In step S9, the micro-processor 33 stops the voltage controlledoscillator from oscillating. Accordingly, the rotor 6 stops rotating.

In the first embodiment, two bearings 16, 17 are used for supporting thespindle 13. However, the spindle can be supported by single bearingarrangement 16. Referring now to FIG. 6, a second embodiment of theinvention is disclosed. The following explanation will be simplified byusing the same reference numerals as the first embodiment.

The spindle 13 has portions 13c, 13d inserted into the bearing 16. Screwthreads are formed on the surface of the slender portion 13d and a nut18 is threadly engaged therewith. The bearing 16 is fixed to the base 1.The spindle 13 is supported by securing the bearing 16 between a portion13e of the spindle 13 and the nut 18.

In the second embodiment, the housing does not require large structuralstrength, therefore, the weight of the housing 15 can be decreased byreducing the thickness.

As described above, each of the first and second embodiments of theultrasonic motor has the following advantages.

Variation of the load torque can be detected mechanically by thepressure modulating mechanism. The pressure by which the rotor 6 ispressed to the stator 2 can be adjusted automatically by the pressuremodulating mechanism so as to reduce the slipping between the rotor 6and the stator 2, if the load torque is increased.

In addition, the variation of the torque load can be electricallydetected by the detecting electrode 4c and the pressure modulatingmechanism. The frequency for generating the traveling wave can beadjusted automatically by the control circuit 30 so as to increase theamplitude of the traveling wave when the load torque is increased.Therefore, output torque can be increased without the rotational speedbeing substantially decreased.

Thus the rotational speed can be maintained not only mechanically butalso electrically against the variation of the load torque.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing application. Theinvention which is intended to be protected herein should not, however,be construed as limited to the particular forms disclosed, as these areto be regarded as illustrative rather than restrictive. Variations andchanges may be made by those skilled in the art without departing fromthe spirit of the present invention. Accordingly, the foregoing detaileddescription should be considered exemplary in nature and not limited tothe scope and spirit of the invention as set forth in the appendedclaims.

What is claimed is:
 1. An ultrasonic motor having a rotor and a statorcomprising:spring means for pressing the rotor to the stator with apredetermined pressure; modulating means for increasing the pressure ofthe rotor to the stator in response to an increase of load; oscillatingmeans for generating a traveling wave on the stator, detecting means fordetecting amplitude of the traveling wave generated on the stator; andwherein the oscillating means controls amplitude of the traveling waveby changing frequency with respect to resonance frequency of the statorwhile the amplitude of the traveling wave is changed.
 2. An ultrasonicmotor in accordance with claim 2, wherein said modulating meanscomprises a first cam surface, a second cam surface and a plurality ofball elements retained therebetween for movement between said first andsecond cam surfaces.
 3. An ultrasonic motor in accordance with claim 2,wherein said second cam surface is driven by rotation of said rotor. 4.An ultrasonic motor in accordance with claim 2, wherein an output drivespindle is driven by said first cam surface, said ball elementstransmitting torque between said first and second cam elements.